1. Technical Field
The present disclosure relates to orthopedic surgical devices and, more particularly, to a spinal stabilization system.
2. Background of Related Art
The spinal column is a complex system of bones and connective tissues that provide support for the human body and protection for the spinal cord and nerves. The adult spine is comprised of an upper and lower portion. The upper portion contains twenty-four discrete bones, which are subdivided into three areas including seven cervical vertebrae, twelve thoracic vertebrae, and five lumbar vertebrae. The lower portion is comprised of the sacral and coccygeal bones. The cylindrical shaped bones, called vertebral bodies, progressively increase in size from the upper portion downwards to the lower portion.
An intervertebral disc along with two posterior facet joints cushion and dampen the various translational and rotational forces exerted upon the spinal column. The intervertebral disc is a spacer located between two vertebral bodies. The facets provide stability to the posterior portion of adjacent vertebrae. The spinal cord is housed in the canal of the vertebral bodies. It is protected posteriorly by the lamina. The lamina is a curved surface with three main protrusions. Two transverse processes extend laterally from the lamina, while the spinous process extends caudally and posteriorly. The vertebral bodies and lamina are connected by a bone bridge called the pedicle.
The spine is a flexible structure capable of a large range of motion. There are various disorders, diseases and types of injury, which restrict the range of motion of the spine or interfere with important elements of the nervous system. The problems include, but are not limited to, scoliosis, kyphosis, excessive lordosis, spondylolisthesis, slipped or ruptured discs, degenerative disc disease, vertebral body fracture, and tumors. Persons suffering from any of the above conditions typically experience extreme or debilitating pain and often times diminished nerve function. These conditions and their treatments can be further complicated if the patient is suffering from osteoporosis, or bone tissue thinning and loss of bone density.
Spinal fixation apparatuses are widely employed in surgical processes for correcting spinal injuries and diseases. When the disc has degenerated to the point of requiring removal, there are a variety of interbody implants that are utilized to take the place of the disc. These include interbody spacers, metal cages and cadaver and human bone implants. In order to facilitate stabilizing the spine and keeping the interbody in position, other implants are commonly employed, such as bone screws and rods. Depending on the pathology and treatment, a surgeon will select the appropriate spinal rod material and size, specifically, the cross-sectional diameter.
To meet the problem of providing a rigid pedicle screw and rod construct, especially for addressing the demands of stiff deformity corrections, larger rod constructs have been made to improve the strength of the screw and rod construct. However when large deformity corrections need to be made, these rods are not always strong enough. Larger diameter stainless steel rods have been made for these applications, but a larger rod requires a larger mating screw head to contain the rod, which in turn increases the profile of the construct. In addition, in order to reduce the likelihood of material incompatibility in vivo, the screw assembly also needs to be made of stainless steel to match the rod material, which is not a cost effective alternative.
Therefore, a need exists for a cost effective, rigid screw and rod construct that can still maintain a low profile.